1. Field of the Invention
The present invention relates generally to ink-jet printing and, more specifically to ink-jet pen alignment using test pattern analysis in a hard copy apparatus"" self-test mode.
2. Description of Related Art
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, see e.g., Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
An ink-jet pen includes a printhead which consists of a number of columns of ink nozzles. The nozzles are employed by printhead drop generating devices (generally thermal, piezoelectric, or wave propagation types) to fire ink droplets that are used to create a printed dots on an adjacently positioned print media as the pen is scanned across the media (for convenience of description, all print media is generically referred to as xe2x80x9cpaperxe2x80x9d hereinafter). Generally, the pen scanning axis is referred to as the x-axis, the print media transport axis is referred to as the y-axis, and the ink drop firing direction from pen to paper is referred to as the z-axis. Within the columns of nozzles, groups of nozzles, called primitives are used to form nozzle arrays grouped by ink color, e.g., four primitives within a column for cyan, yellow, magenta, or black ink (xe2x80x9cCYMKxe2x80x9d). A given nozzle of the printhead is used to address a given vertical column position on the paper, referred to as a picture element, or xe2x80x9cpixel,xe2x80x9d where each nozzle-fired drop may be only a few picoliters (10xe2x88x9212 liter) in volume and the resultant ink dot only {fraction (1/600)}th-inch. Horizontal positions on the paper are addressed by repeatedly firing a given nozzle as the pen is rapidly scanned across the adjacent paper. Thus, a single sweep scan of the pen can print a swath of dots generally equivalent to the nozzle column height. Dot matrix manipulation is used to form alphanumeric characters, graphical images, and photographic reproductions from the ink drops. The print media is stepped in the y-axis to permit a series of scans, the printed swaths combining to form text or images.
In general, ink-jet hard copy apparatus are provided with two to four pens; either a set of three single color pens, or a single pen with three colorant reservoirs and at least three primitives, and a black ink pen. It is also known to print composite black using color ink. Static pen, and hence printhead nozzle alignment, is a function of the mechanical tolerances of the scanning carriage mounts for the individual pens. Moreover, ink-jet writing systems with reciprocating carriages typically have inherent dot placement errors associated with the dynamics of carriage motion. Such errors are usually associated with vibrations and therefore are cyclical in nature. If printing with a constant carriage velocity, these errors will manifest themselves on the paper at regular spatial pitches across the width of the page. Thus, among other factors, the pitch of the error will be a function of carriage velocity.
One method for determining and correcting nozzle-firing algorithms for pen alignment error parameters is where a hard copy apparatus prints a test pattern and uses the test pattern to determine the pen alignment error parameters. [Note that nozzle firing manipulation via computerized program routines, xe2x80x9calgorithms,xe2x80x9d is a complex art in and of itself. While knowledge in that field is helpful, it is not essential to an understanding of the present invention which relates to printing error parameter derivations subsequently used by such nozzle firing algorithms.] Many such systems require the end, user to inspect a variety of patterns visually and to select the pattern, and hence the hard copy apparatus settings, which are most appealing to that individual.
In U.S. Pat. No. 5,250,956, Haselby et al. use a test pattern for print cartridge bidirectional alignment in the carriage scanning axis; in U.S. Pat. No. 5,297,017, Haselby uses a test pattern for print cartridge alignment in the paper feed axis.
In U.S. Pat. No. 5,262,797, Boeller et al. disclose a standard pen plotter related method of monitoring and controlling quality of pen markings on plotting media in which an actual line plot is optically sensed across a selected point to make a comparison with a test line.
In U.S. Pat. No. 5,289, 208, Haselby discloses an automatic print cartridge alignment sensor system.
In U.S. Pat. No. 5,448,269, Beauchamp et al. use a test pattern for multiple ink-jet cartridge alignment for bidirectional printing.
In U.S. Pat. No. 5,451,990, Sorenson et al. use specified test patterns as a reference for aligning multiple ink-jet cartridges.
In U.S. Pat. No. 5,600,350, Cobbs et al. teach multiple ink-jet print cartridge alignment by scanning a reference pattern and sampling the same with reference to a position encoder.
[Each patent listed above is assigned to the common assignee of the present invention. It is also known to use test patterns for testing and clearing of nozzles, testing ink quality, and for color correction; those functions are beyond the scope of the present invention and require no further explanation for an understanding of the present invention.]
Generally, large format ink-jet plotters use the strategy of using one block of nozzles from one column on one printhead as a reference. All other nozzles on every printhead are then aligned relative to this reference block.
There remains a need in the state-of-the-art for more accurate methodologies for aligning ink-jet printheads. There remains a need for automatic alignment of ink-jet printheads, that is, without the need for reliance on the user""s visual acuity. There remains a need for techniques for avoiding carriage-induced dynamic errors during automated alignment of ink-jet printheads. There remains a need for test patterns for use in automated alignment of ink-jet printheads which are suited to providing a variety of printhead alignment information in a compact format.
In its basic aspects, the present invention provides a method of determining ink-jet printhead alignment offset. The method includes the steps of: printing a test pattern on a sheet of media, the test pattern providing a design of predetermined nominal shape and spacing parameters in accordance with a first data set; acquiring a second data set representative of actual shape and spacing parameters of the test pattern from the test pattern on the sheet of media; fitting a first waveform representative of the first data set to the second data set such that an initial fit offset value is determined by a characteristic of fit between the first waveform and the second data set; partitioning the second data set into a plurality of individual third data sets selectively chosen from the pattern for measuring differential offset values evidenced in the second data set; fitting a measuring construct to each of the individual third data sets for determining an actual printhead alignment offset value for each of the third data sets; and calculating an actual printhead alignment offset value for each of the third data sets using the initial offset in combination with comparison data representative of comparing the measuring construct and the second data set.
In another basic aspect, the present invention provides a computer memory for implementing an automatic alignment of an ink-jet printhead device. The memory includes program routines for storing a test pattern first data set, the test pattern having objects with given nominal object spacing and object width; program routines for storing a test pattern second data set from reading back a printed first test pattern data set; program routines for fitting a first waveform representative of the first data set to the second data set such that an initial fit offset value is determined by a characteristic of fit between the first waveform and the second data set; program routines for partitioning the second data set into a plurality of individual third data sets selectively chosen from the pattern for measuring differential offset values evidenced in the second data set; program routines for fitting a measuring construct to each of the individual third data sets for determining an actual printhead alignment offset value for each of the third data sets; and program routines for calculating an actual printhead alignment offset value for each of the third data sets using the initial offset in combination with comparison data representative of comparing the measuring construct and the second data set.
In another basic aspect the present invention provides a method for aligning ink-jet printhead devices in a hard copy apparatus having a printhead nozzle-firing mechanism for directing ink-jet nozzle firing pulses. The method includes the steps of: upon changing at least one of the printhead devices or upon an end-user apparatus test mode implementation command, automatically printing on a print media a given test pattern from a first data set having test pattern objects of a given shape and spacing dimensions, the given test pattern including objects relevant to determining printhead device alignment offset values relative to the at least one of the devices; automatically reading back printed test pattern information as a second data set; partitioning the second data set into a plurality of subpatterns representative of printing in a predetermined orientation such that a plurality of subpattern offset values is represented for the printing in a predetermined orientation; fitting a measuring construct to each of the subpatterns; determining from the measuring construct a printhead device alignment offset value between a printed test pattern object actual position and a printed test pattern object expected position based upon the first data set; and transmitting a final printhead device alignment offset value based upon the initial offset and the printhead device alignment offset value to the printhead nozzle-firing mechanism.
It is an advantage of the present invention that it provides a unified method for measuring various systematic ink-jet printhead misalignment characteristics and parameters.
It is an advantage of the present invention that it provides an alignment correction factor having a greater resolution than previous methodologies.
It is another advantage of the present invention that an offset value correction as small as one-eighth of a printed dot diameter can be achieved.
It is another advantage of the present invention that it provides a computerized process which calculates alignment error values with minimal computational requirements.
It is a further advantage of the present invention that it provides a computerized, automated alignment error correction, requiring no visual perception assessment and comparison reassessment by the end-user of a variety of test patterns.
It is a further advantage of the present invention that it can be automatically implement upon a printhead change or user implemented, e.g., when changing print media.
It is an advantage of the present invention that it provides a test pattern plot that is quickly printed and analyzed using only one sheet of A-size paper.
It is an advantage of the present invention that it provides a test pattern plot which minimizes the need to print with one column of reference nozzles only.
It is an advantage of the present invention that it provides a test pattern plot wherein the printhead alignment process is less sensitive to defects in one particular reference block of nozzles.
It is another advantage of the present invention that it provides a test pattern which provides extensive data used to compensate for harmonic frequency carriage motion induced printing errors.